1. Field of the Invention
This invention relates to an optical wavelength converter system for shortening the wavelength of a laser beam, and more particularly, to an optical wavelength converter system in which a second harmonic wave is produced by the introduction of a laser beam into the crystal of a first nonlinear optical material and then a sum frequency is produced by the introduction of the laser beam together with the second harmonic wave into the crystal of a second nonlinear optical material.
2. Description of the Prior Art
An example of such a converter system is already known from, for example, U.S. Pat. No. 4,701,929 which discloses a laser-diode-pumped solid-state laser in which a solid-state laser rod doped with rare earth materials such as neodymium is pumped by a semiconductor laser, i.e., a laser diode. In order to obtain a laser beam having a shorter wavelength, in the case of this type of solid-state laser, a bulk single crystal of nonlinear optical material is disposed inside of the resonator of the laser for converting the wavelength of the beam, which is oscillated by the solid-state laser, into the wavelength of a second harmonic wave.
For the purpose of producing a laser beam having a much shorter wavelength, for instance, in a range of UV, as disclosed in "Technical Digest of Conference on Lasers and Electro-Optics" in 1991, pp. 218, attempts have been made to produce a sum frequency by the introduction, into a crystal of a second nonlinear optical material, of a laser beam oscillated by a solid-state laser and a second harmonic wave of the laser beam.
More specifically, in this type optical wavelength converter system, a YAG crystal, which is a medium for a solid-state laser, is pumped by light from a light source to produce a solid-state laser beam at a wavelength of 1064 nm, and then this solid-state laser beam is introduced into a KTP crystal which serves as a first nonlinear optical material to produce a second harmonic wave at a wavelength of 532 nm. The second harmonic wave thus obtained and the aforesaid solid-state laser beam are further introduced into a BBO crystal which serves as a second nonlinear optical material to produce a sum frequency at a wavelength of 355 nm.
This type of optical wavelength converter system is deemed useful in producing a laser beam with an ultra short wavelength, but inferior because of its very low wavelength conversion efficiency. This allows the converter system to produce only a pulse-like laser beam which permits an increase in power inside of the resonator, but not to produce a continuous laser beam which causes a lower inside power. As a specific example, when the oscillation of a solid-state laser was Q-switched with the use of a light source for the pump use, having an output of 5 kW, so that the power inside of the resonator could be increased and when two fundamental waves were phase matched, in type-I, with a sum frequency, the output of the sum frequency was reported to be 100 mW.